PPARalpha: A new PPARalpha humanized mouse line was produced in which the human receptor is expressed from the native human gene in order to recapitulate in the mouse the regulation and tissue-specific expression of the human PPARalpha. PPARalpha-humanized transgenic mice were generated using a P1 phage artificial chromosome (PAC) genomic clone bred onto a PPARalpha-null mouse background, designated hPPARalpha(PAC). In hPPARalpha(PAC) mice, the human PPARalpha gene is expressed in tissues with high fatty acid catabolism and induced upon fasting, similar to mouse PPARalpha in wild-type mice. Upon treatment with the peroxisome proliferator fenofibrate, hPPARalpha(PAC) mice exhibited responses similar to wild-type mice, including peroxisome proliferation, lowering of serum triglycerides and induction of PPARalpha target genes encoding enzymes involved in fatty acid metabolism in liver, kidney and heart, suggesting that human PPARalpha functions in the same manner as mouse PPARalpha in regulating fatty acid metabolism and lowering serum triglycerides. However, in contrast to wild-type mice, treatment of hPPARalpha(PAC) mice with fenofibrate did not cause significant hepatomegaly and hepatocyte proliferation, thus indicating that the mechanisms by which PPARalpha affects lipid metabolism are distinct from the hepatocyte proliferation response, the latter of which is only induced by mouse PPARalpha. The hPPARalpha(PAC) mouse model provides an in vivo platform to investigate the species difference mediated by PPARalpha and an ideal model for human risk assessment peroxisome proliferators exposure. To determine the mechanism of species differences in responses to peroxisome proliferators and the mechanism of peroxisome proliferator-induced hepatocarcinogenesis, microarray analysis was done on Wy-14,643 (a PPARalpha activator) treated wild-type and hPPARalpha (TetOFF) mice, sensitive and resistant, respectively to hepatocyte proliferation and liver cancer. A novel mechanism by which PPARalpha regulates gene expression and hepatocellular proliferation was uncovered. MicroRNA (miRNA) expression profiling demonstrated that activated PPARalpha was a major regulator of hepatic miRNA expression. Of particular interest, let-7C, an miRNA important in cell growth, was inhibited following 4-h treatment and 2-week and 11-month sustained treatment with the potent PPAR_ agonist Wy-14,643 in wild-type mice. let-7C was shown to target c-myc via direct interaction with the 3 untranslated region of c-myc. The PPAR_-mediated induction of c-myc via let-7C subsequently increased expression of the oncogenic mir-17-92 cluster; these events did not occur in PPARalpha-null mice. Overexpression of let-7C decreased c-myc and mir-17 and suppressed the growth of Hepa-1 cells. Furthermore, using the human PPARalpha-expressing mouse model, which is responsive to Wy-14,643 effects on beta-oxidation and serum triglycerides but resistant to hepatocellular proliferation and tumorigenesis, we demonstrated a critical role for let-7C in liver oncogenesis. Wy-14,643 treatment did not inhibit let-7C or induce c-myc and mir-17 expression. These observations reveal a let-7C signaling cascade critical for PPAR_ agonist-induced liver proliferation and tumorigenesis. PXR: The most common clinical implication for the activation of the human PXR is the occurrence of drug-drug interactions mediated by up-regulated cytochromes P450 3A (CYP3A) isozymes. Typical rodent models do not predict drug-drug interactions mediated by human PXR because of species differences in response to PXR ligands. In the current study, a PXR-humanized mouse model was generated by bacterial artificial chromosome (BAC) transgenesis in PXR-null mice using a BAC clone containing the complete human PXR gene and 5'- and 3'-flanking sequences. In this PXR-humanized mouse model, PXR is selectively expressed in the liver and intestine, the same tissue expression pattern as CYP3A. Treatment of PXR-humanized mice with the PXR ligands mimicked the human response, since both hepatic and intestinal CYP3As were strongly induced by rifampicin, a human-specific PXR ligand, but not by pregnenolone 16alpha-carbonitrile, a rodent-specific PXR ligand. In rifampicin-pretreated PXR-humanized mice, an approximately 60% decrease was observed for both the maximal midazolam serum concentration (Cmax) and the area under the concentration-time curve, as a result of a 3-fold increase in midazolam 1'-hydroxylation. These results illustrate the potential utility of the PXR-humanized mice in the investigation of drug-drug interactions mediated by CYP3A and suggest that the PXR-humanized mouse model would be an appropriate in vivo tool for evaluation of the overall pharmacokinetic consequences of human PXR activation by drugs. Rifaximin, a rifamycin analog approved for the treatment of travelers' diarrhea, is also beneficial in the treatment of multiple chronic gastrointestinal disorders. However, the mechanisms contributing to the effects of rifaximin on chronic gastrointestinal disorders are not fully understood. In the current study, rifaximin was investigated for its role in activation of the PXR, a nuclear receptor that regulates genes involved in xenobiotic and limited endobiotic deposition and detoxication. PXR-humanized (hPXR), PXR-null, and wild-type mice were treated orally with rifaximin, and rifampicin, a well characterized human PXR ligand. Rifaximin was highly concentrated in the intestinal tract compared with rifampicin. Rifaximin treatment resulted in significant induction of PXR target genes in the intestine of hPXR mice, but not in wild-type and PXR-null mice. However, rifaximin treatment demonstrated no significant effect on hepatic PXR target genes in wild-type, PXR-null, and hPXR mice. Consistent with the in vivo data, cell-based reporter gene assay revealed rifaximin-mediated activation of human PXR, but not the other xenobiotic nuclear receptors CAR, (PPAR)alpha, PPARgamma, and FXR. Pretreatment with rifaximin did not affect the pharmacokinetics of the CYP3A substrate midazolam, but it increased the Cmax and decreased Tmax of 1'-hydroxymidazolam. Collectively, the current study identified rifaximin as a gut-specific human PXR ligand, and it provided further evidence for the utility of hPXR mice as a critical tool for the study of human PXR activators. Further human studies are suggested to assess the potential role of rifaximin-mediated gut PXR activation in therapeutics of chronic gastrointestinal disorders. AHR: CYP1A1 is one of the most important detoxification enzymes due to its broad substrate specificity and wide distribution throughout the body. On the other hand, CYP1A1 can also produce highly carcinogenic intermediate metabolites through oxidation of polycyclic aromatic hydrocarbons. We describe what we believe to be a novel regulatory system for whole-body CYP1A1 expression by a factor originating in the gut. A mutant mouse was generated in which the Arnt gene is disrupted predominantly in the gut epithelium. Surprisingly, CYP1A1 mRNA expression and enzymatic activities were markedly elevated in almost all non-gut tissues in this mouse line. The induction was even observed in early-stage embryos in pregnant mutant females. Interestingly, the upregulation was CYP1A1 selective and lost upon administration of a synthetic purified diet. M [summary truncated at 7800 characters]